1. Field of the Invention
The present invention relates to control valves for a variety of hydraulic devices, and more particularly to a control valve having a variable priority construction integrally equipped in its load hold check valve.
2. Description of the Prior Art
Where at least two actuators are being operated in a combined manner by oil delivered from a single pump, a "priority" is established to control the actuators such that one of the actuators is supplied with a larger amount of oil than the other actuator. For example, excavators have a priority of the swing actuator over the arm actuator and a priority of the boom over the bucket. The reason why such priorities are given is because in most cases, the amount of oil required for a swing operation is larger than the amount of oil required for an arm operation, and the amount of oil required for a boom operation is larger than the amount of oil required for a bucket operation. As oil is supplied in different amounts depending on the kind of operation in accordance with the priorities, it is possible to prevent unnecessary loss of pressure and achieve a smooth operation.
Conventionally, means for providing such a priority function is installed as a separately in a hydraulic circuit or integrally equipped in a control valve.
Referring to FIG. 1, there is shown a conventional priority device installed separately in a hydraulic circuit. As shown in FIG. 1, the priority device includes an orifice 201 disposed in a parallel fluid line 203. The orifice 201 serves to limit the amount of oil supplied to actuator A associated therewith and thereby increasing amount of oil supplied to the swing motor using the limited amount of oil. By such a function of the orifice 201, actuator B has priority over actuator A.
Referring to FIG. 2, there is shown another conventional priority device which is integrally equipped in a control valve. As shown in FIG. 2, the priority device includes an orifice fluid passage 305 formed in a load hold check valve 303 of a control valve unit 301. The orifice fluid passage 305 has a diameter considerably smaller than those of other fluid passages provided at the control valve unit 301. In the control valve unit 301, oil in the parallel fluid passage 307 flows through the orifice fluid passage 305 to the left actuator port 309a or the right actuator port 309b while urging the load hold check valve 303 in a downward direction. When the oil passes through the orifice fluid passage 305 communicating with one of the actuators, a reduction in oil amount is generated because the orifice fluid passage 305 has a diameter considerably smaller than those of other fluid passages provided at the control valve unit 301. As a result, the amount of oil corresponding to the reduced amount of oil is additionally supplied to the other actuator via the parallel fluid passage 307. Thus, the priority function is achieved.
Where two actuators are operated in a combined manner, the above-mentioned conventional priority devices provide a desirable effect by establishing a priority of one actuator over the other actuator. Where only the actuator communicating with the orifice installed in the parallel fluid line or the orifice fluid passage formed in the control valve is operated, however, there are problems of unnecessary loss of pressure and a decrease in the actuating speed of the actuator because the amount of oil supplied to the actuator is always limited.
In order to solve these problems, there has been proposed a variable priority device. Where such a variable priority device is used to obtain a priority of the swing actuator over the arm actuator, it serves to supply a sufficient amount of oil to the arm cylinder by releasing an orifice function when the arm actuator operates alone and to decrease the amount of oil supplied to the arm actuator and relatively increases the amount of oil supplied to the swing actuator by the orifice function, thereby enabling the swing actuator to have priority over the arm actuator. Generally, such a variable priority device includes a variable orifice which is disposed in a parallel fluid line and switched between an orifice state and an orifice release state in response to a given pressure signal.
However, this variable priority device has a drawback that its hydraulic circuit should have complex additional elements such as the variable orifice and a pressure signal generating circuit for switching the variable orifice.